Almost £500m was wiped off the stockmarket value of Sports Direct on Tuesday as the City responded to founder Mike Ashley's decision to offload a £200m tranche of shares.

The retailer was the biggest faller in the FTSE 100, closing down more than 9% after the investment bank Goldman Sachs looked to place 24m shares it bought from the entrepreneur on Monday.

CooperCity analyst Louise Cooper suggested big investors had decided to express derision following a recent showdown over a bonus scheme that could have handed Ashley £72m of free shares. "I think the institutions have turned around and given Goldman the two fingers," she said. "It's difficult to know but the market might be saying that Goldman has been left with stock."

Shares in Sports Direct, which have more than doubled in the past year, closed at £8.11, down 82.5p. Ashley still controls a 57.7% stake worth £2.8bn.

The timing of the deal means Ashley has, for a second time, sold shares from his personal investment company MASH within a lock-up period, when he has promised not to deal. In October, when he sold £106m of shares, he promised not to sell any more for 180 days. Only 166 days have elapsed since that pledge.

The October sale was also a vow breaker. In February 2013 Ashley said he would not to sell any more of his stake for at least 12 months after he sold shares worth £100m.

The pledges are not binding – but there is an obligation for a firm to ensure any information it issues is not misleading. It is thought Goldman Sachs agreed to waive the lock-up period because of Sports Direct's recent strong stock market performance, indicating a high level of demand for shares. The retailer is due to update the City on its performance later this month.

Both Goldman Sachs and Sports Direct declined to comment.

The sell-off follows the decision last week by Sports Direct investors – excluding Ashley – to block a proposed bonus share scheme for the billionaire retailer. It is the third time such a payout plan has failed. Ashley, who founded Sports Direct in 1982, has never drawn a salary from the listed firm. It also comes as investors try to understand the tactics behind Sports Direct's surprise purchase of an 11% stake in House of Fraser, which emerged at the weekend. Ashley, Sports Direct's executive deputy chairman, paid around £20m for Sir Tom Hunter's stake in the department store chain after China's Sanpower Group agreed to acquire 89% in a deal worth more than £450m, including debt.

In a statement on Tuesday, Nanjing Xinjiekou Department Store, the listed retail arm of Sanpower, said it would sign the deal shortly and would set up a wholly owned subsidiary in the UK to acquire the stake. Ashley is known to have long held an interest in buying House of Fraser, although any attempt to derail Sanpower's move looks to have failed, with the transaction expected to be completed this week.

In January Ashley kicked off the new year with another surprise move, selling a stake he just bought in UK department store chain Debenhams for a profit of about £4.6m and betting on its share price holding up with a complex option deal.

Insiders said his sale of 24m Sports Direct shares at £8.50, which was confirmed on Tuesday, was not linked to the House of Fraser acquisition. Instead they pointed to calls from investors to increase the company's free float.